As a new field of displays, electronic books, PNDs (personal navigation devices), PDAs (personal digital assistants), and other types of display devices carried by individuals are becoming more popular. Further, display devices like electronic newspapers using electronic paper which can be read by the same feeling as with conventional printed paper are being developed. Organic EL (electroluminescence) displays and display panels using electronic paper are recently particularly coming under the spotlight. These displays have to mount drive use TFTs (thin film transistors) in order to achieve high performance display. The current mainstream of TFTs are Si-based semiconductor materials. These are usually subjected to process temperatures of 300 to 450° C. due to annealing etc. The TFTs are therefore fabricated on glass substrates.
In recent years, for popularizing the new portable display terminals and electronic paper, displays which, unlike conventional displays, are thin, light, and flexible like paper are being sought. Safety, i.e., not breaking even if dropped, is also being sought. With conventional glass substrates, these demands cannot be met. Therefore, display substrates using organic plastic films or metal foil are being developed.
For example, electronic paper is produced by combining electronic ink comprised of 50 to 70 μm diameter microcapsules in which charged black and white particles are sealed, that is, “E Ink”, and 0.1 to 0.2 mm or so thickness TFT substrates. A front substrate formed by thin transparent plastic film is coated with transparent electrodes and microcapsules in which black and white particles are sealed. A back substrate is formed with switches for applying voltage comprised of TFT circuits. Due to the black and white particles in the microcapsules, by applying voltage between the transparent electrodes at the front substrate and the electrodes at the back substrate, the capsules gather at one, thereby enabling black and white display. For the front substrate, a plastic film can be used, but the back substrate for forming the TFTs requires heat resistance, chemical resistance, etc., so selection of the substrate material is difficult.
An organic plastic film is light in weight, superior in flexibility, and diverse in type. However, in applications for TFT substrates for display use, chemical stability, heat resistance, moisture resistance, etc. sufficient to withstand the TFT fabrication process are demanded, so the resins which can be used are limited. Polyimides, PES, PEN, etc. are being studied [see NPLTs 1, 2], but there are no plastic films able to be widely used in 350° C. or more processes giving good TFT characteristics.
In metal foil, stainless steel foil superior in heat resistance and corrosion resistance is being studied as a TFT substrate and prototype displays are being made [see NPLTs 3 to 5]. Metal foil is conductive, so when using it as a display substrate instead of a glass substrate, it is necessary to form an insulating film on its surface. Attempts are being made to use insulated stainless steel foil as a substrate for fabricating TFTs [see PLT 1].
The resolution of display elements of electronic paper etc. is governed by the precision of fabrication of the TFTs, so in general there is an extremely strong demand for surface smoothness of glass substrates for display use. For example, specifications demand an Ra of 15 μm square region observed by an atomic force microscope (AFM) of 5 nm or less. For an insulated stainless steel foil as well, the surface of the insulating film is required to have a high smoothness. The thickness of the insulating film is usually around 2 μm, so the smoothness of the surface of the stainless steel foil itself also has an effect on the smoothness of the surface of the insulating film. Therefore, as a method for flattening the surface of stainless steel foil, after foil rolling, bright annealing is performed to give the surface a mirror finish. A bright annealed material is called a “BA material” and is superior in flatness to a hard (H) material which is not bright annealed. FIG. 1 gives scan electron microscope (SEM) photographs of the surfaces of an H material and BA material of SUS430 and SEM photographs of the surfaces of the insulating films after forming insulating films on the same to a thickness of 1.5 μm. The H material of SUS430 exhibits noticeable stripes parallel to the rolling direction called “ridging”. Even if forming a 1.5 μm insulating film, these stripes remain as clear from the photographs. The Ra's measured by a stylus type surface roughness meter were 61 nm and 15 nm for the H and BA treated surfaces of stainless steel foil, while the roughness Ra of the surfaces of the insulating films were 28 nm and 11 nm. A BA material is higher in flatness than an H material. The smoothness of the insulating film formed on it is also superior, it is learned.
However, stainless steel foil of a bright annealed BA material, while also depending on the thickness, has the problem of a poor shape recovery. When rolling up foil of a thickness of about 150 μm to a tube of a size of a diameter of about 50 mm able to be held by one hand, the foil will not return to a flat surface even after the rolling force is removed. If trying to forcibly return its shape, an uneven surface results. A BA material is believed to be poor in shape recovery due to the removal of strain and softening during annealing. For a flexible display substrate, whether the original shape is returned to after rolling is important. Unless able to be spread flat and viewed after being rolled up, use as a display is difficult. Therefore, a TFT back substrate for flexible display use is required to have shape recovery after being rolled up, but up to now no method for achieving both surface smoothness and shape recovery has been discovered.